Many polymers undergo an endothermic phase change within a specific temperature range. There are several types of such phase-change polymers. Low-melting polymers such as Poly(ethylene glycol), pluronic and Poly(caprolactone) undergo a melting transition at temperatures ranging from 15° C. to 60° C. Another class of polymers are the temperature-responsive polymers, that undergo a coil-to-globule transition at critical temperatures. For example, such polymers may undergo a phase change at a critical temperature known as the Lower Critical Solution Temperature (LCST) or at a critical temperature known as the Upper Critical Solution Temperature (UCST). At the LCST polymers transition from a single phase into a two-phase system. Such polymers include Poly(N-isopropylacrylamide), Hydroxypropyl methylcellulose (HPMC), and Poly (diethylacrylamide), among others. The LCST can also be observed for thermoresponsive polymers in the solid state (Liu and Urban, Macromolecules, 42(6) pp. 2161-2167, 2009). A critical temperature for phase change can be adjusted to a desired range through copolymerization with more hydrophilic polymers or hydrophobic polymers to increase or decrease the temperature, respectively. Some polymers are known to undergo a coil-to-globule transition, which is an endothermic phase transition and leads to significant heat absorption, generally in the range of about 50-200 J/g.
Many phase-change materials (PCMs) are known and have been used for thermoregulation, e.g., for keeping various articles within a desired temperature range or cooling the skin temperature of the human body. However, while maximum heat absorption can be achieved through an endothermic melting transition, known PCMs are unsuitable for application on articles such as textiles or packaging materials without an encapsulating agent, due to a need to contain the liquid produced by solid-liquid transitions. Thus, formulations used for thermoregulation typically include microencapsulating agents, such as urea-formaldehyde microcapsules, to contain liquid after melting. As these microcapsules do not naturally adhere to many substrates, such as textile or packaging substrates, fixative agents to promote adherence to substrates are further required.
These thermoregulatory formulations have several disadvantages. The presence of multiple components (e.g., microencapsulating agents, fixative agents) in a thermoregulatory formulation significantly reduces the proportion of pure PCM in the formulation and hence reduces its heat capacity. Furthermore, there is a limit to the quantity of a formulation that can be loaded on a surface, so that the presence of other components limits the amount of PCM that can be adhered to a surface. For example, phase change materials such as paraffin and salt hydrates absorb well over 200 kJ/kg, but encapsulation and inclusion as a textile finishing or co-spinning with cellulosic or polyester fabrics reduces their heat capacity to as little as 5 J/g in some cases (see, e.g., European Patent Application Publication No. P1846598A1). The need to saturate some substrates, such as textiles, with these PCMs has the effect of increasing thermal resistance of a textile, thus cancelling out the beneficial heat-absorbing effects of the PCM. In addition, known PCMs are short-lived, providing heat absorption for no longer than about 10 minutes, as well as flammable and/or irritating to the skin in the case of garments, or are too toxic to be in contact with food and biological products, in the case of packaging applications.
Use of nanocrystalline particles to improve mechanical properties of PCMs and to obtain solid-solid phase transitions has been reported. Yuan et al. (Yuan et al., Chinese Chemical Letters, Vol. 17, No. 8, pp 1129-1132, 2006) grafted PEG chains onto nanocrystalline particles to avoid the need to encapsulate the phase change material, and solid-solid phase transitions were obtained. However, the heat absorption capacity of the resulting nanocrystalline particles was far lower than the capacity of the starting material, resulting in poor performance as compared to encapsulated products already available. Such nanocrystalline particles therefore fail to overcome the limitations of existing PCMs.
There is a need therefore for improved PCMs that display a desired heat absorbing capacity, maintain their solid state form during phase transition, and/or do not require high amounts of fillers such as encapsulating agents or fixatives, in order to provide energy-dense PCMs and thermoregulatory formulations that maximize heat absorption using minimal quantities of material.